With recent development of cloud computing and increase of smartphone users, communication traffic is ever-increasing. This requires an enormous amount of electricity in data servers where transmitted information data is gathered. Moreover, their processing capacity is reaching the limit. There is an urgent need for development of technologies for improving such problems. One of the actively studied technologies is opto-electronic circuit boards (also called opto-electronic hybrid boards) which change part of electrical interconnects in a server board to optical interconnects, and this technology enables data processing at high density and at high speed.
The opto-electronic circuit board requires an optical waveguide as an optical transmission path as well as a photoelectric transducer converting an electrical signal into an optical signal. Techniques including vertical cavity surface emitting layers (VCSEL) and silicon photonics are known as light sources for the photoelectric transducer. Especially, silicon photonics developed from an application of semiconductor processes, such as CMOS and MEMS, has become a mainstream. The wavelengths of light transmitted have changed from 850 nm for VCSEL to the long wavelength range, such as the wavelength of 1,310 nm and the wavelength of 1,550 nm, for silicon photonics.
To incorporate electrical interconnects, a solder reflowing process up to about 300° C. is required, and the optical waveguide should have heat resistance to the process.
There is a need for optical waveguides that have a low transmission loss in the above-noted long wavelength range and have heat resistance to 300° C. or higher, and preferably 350° C. or higher. A vinyl-based silicone compound is disclosed as a material that can form such optical waveguides (Patent Document 1).
Optical waveguides are classified in accordance with their structure mainly into two types: step-index (SI) and graded-index (GI) structures. The SI structure has been employed in conventional optical waveguides in view of its processability. In the SI structure, a core and cladding define an interface between refractive indices, and reflection at the interface allows light to propagate. On the other hand, in the GI structure, the refractive index is highest at the core center and gradually decreases outward. This structure allows light to be guided and propagate only in the vicinity of the core center. The GI structure is free from crosstalk even with cores with narrow pitches and is thought to be an ideal optical waveguide shape.
Known materials used in few reports on GI optical waveguides include certain norbornene resins and epoxy compounds (Patent Document 2). Light is applied to a coating film of this resin to induce mass diffusion of the photolyte and the photocurable epoxy compound, thereby producing a refractive index gradient.